Mobility is too strong in league right now. The issue is multifaceted, and there is one side that has overshadowed all the others, that being the prevalence of it. However, there is a hidden side that is regularly seen impacting games that needs to be addressed:
**Preventing mobility from happening is too difficult even when applying the correct CC.**
For example, let's look at Tristana, but this scenario is valid for almost all champions because of how league works: Let's say Sion catches Tristana in a Q. He gets the 1 second charge, so she is officially hosed unless she can squeeze in an ult before he releases it. But not really, because it is actually still in her favor to escape with W! When she casts it, if Sion releases Q in response, she still gets away. He has to wait until she is actually moving through the air before applying his CC, or the hidden power of W's cast time will allow her to escape.
This ~~bug~~ feature is actually doing a lot to make the mobility situation far worse because the limited CC available to champions is being correctly applied, enemies are being caught in it, but the effect is being countered without any cost. All forms of immobilizing CC need to be rewritten to prevent a champion's movement during the duration of the CC, and spells with movement that are being cast at the time CC is applied should simply be countered.
Thank you for your time.